A network in accordance with the present invention may include certain items that are common as to their functions indicated below with a first known network. These items include:
a plurality of terminals associated with respective user peripherals between which messages are to be transmitted, each of the terminals being suitable for emitting an emission wave at a controlled emission frequency which constitutes a position of the terminal within a spectrum range of the network, the terminal also being suitable to apply information-carrying modulation to said wave so as to cause it to convey one of said messages; PA1 two transmission lines associated with each of said terminals to guide waves and which are constituted by an emission line for guiding said emission wave from said terminal and a reception line for guiding waves received by said terminal; PA1 a star coupler for receiving said emission waves over said emission lines and for transmitting each of said waves to all of the terminals via said reception lines such that the positions of the various terminals must remain at mutual distances apart that are not less than a predetermined frequency increment; and PA1 frequency allocation means for allocating positions to the terminals for the calls in which they are to participate.
The emission frequencies of the various terminals in such a network vary over time. But to avoid such variations giving rise to cross-talk, the spectrum distances between emission frequencies must remain at not less than one frequency increment. The frequency increment is selected to reserve a message channel of adequate bandwidth for each call within the spectrum range of the network. Starting from a base frequency, these frequencies thus constitute a sequence which is referred herein as a "stack" and which extends up to a top of the stack. This sequence corresponds to the sequence of message channels including said frequencies and also the sequence of terminals utilizing said frequencies. In each of these sequences an item may be adjacent to another item, i.e. it may precede it or it may follow it. The top of the stack is constituted by one of the emission frequencies or, for a call established over a plurality of frequencies, by a group of such frequencies being used for the same call and which are adjacent, for example, said frequency or group of frequencies being the furthest from the base frequency.
The various terminals may be identical to one another. That is why, to facilitate the description of the operation of such a network consideration is sometimes given more particularly to one of the terminals, which may be constituted by any one of them and which is referred to as the "terminal under consideration".
It will be understood that the word "wave" used herein is capable of designating waves or signals that may be at various different frequencies, and that may be of various different kinds, e.g. electromagnetic or acoustic. Nevertheless, the present invention relates more particularly, but not exclusively, to the case where said emission waves are optical waves and where said transmission lines are optical fibers for guiding said waves.
The said first known network constitutes one such network. It is described in a first prior document constituted by European patent Document EP-A-381 102 (F*16761). That document explains how the emission frequency of the terminal under consideration is established as it prepares to begin a call. The frequency is established by a "support". Such a "support" consists in the terminal under consideration establishing its emission frequency relative to a support frequency in such a manner that the spectrum distance between said emission frequency and its support frequency is equal to one of said frequency increments. A support frequency is typically constituted by the emission frequency of another terminal.
To implement such support, the terminal under consideration scans the spectrum range of the network. Such scanning enables it to identify possible emission frequencies that are not busy and that are therefore available for emitting the message to be transmitted. An emission frequency identified in this way is selected and is used initially to send calling signalling which includes, at least, an address for the called terminal. The duration of such scanning increases the access time of terminals to the network, i.e. the time which elapses on average between the instant when the terminal receives an instruction from the associated user peripheral to transmit a message and the instant at which said transmission begins.
A second known network is described in a second prior document constituted by an article "Protocols for very high-speed optical fiber local area networks using a passive star topology" (Isam M. I. Habbab, Mohsen Kavehrad, and Carl-Erik W. Sundberg, Journal of Lightwave Technology, Vol. LT-5, No. 12, December 1987, pp. 1782-1793).
The spectrum range of said second known network includes a plurality of message channels and a signalling channel and the positions of the channels appear to be predetermined. The signalling channel is reserved for transmitting signalling making it possible, in particular, for a calling terminal to indicate a message channel to the called terminal for use in a call.
A particular object of the present invention is to limit the access time of a terminal to such a network.